In order to record/erase/reproduce information, at a high speed, to/from a disc-shaped optical recording medium whose substrate is made of a resin, it is necessary to spin the optical recording medium at a high speed.
However, when a disc made of a resin such as polycarbonate or polyolefin whose thickness is about 1.2 mm, e.g., a CD (compact disc), a DVD (digital versatile disc), a BD (blu-ray disc), or the like, is spun at a speed of 10000 rpm or more, the surface deflection may increase due to the insufficient strength, thereby breaking the disc. One may consider using a metal, a glass or a ceramic having a higher rigidity than a resin, but it will result in a high cost. In view of this, a disc whose thickness is about 0.1 mm has been proposed as a method of using a resin (e.g., Non-Patent Document No. 1).
Such a thin disc also has an inner hole and an outer shape that are concentric and circular, as those of a conventional disc, and the inner hole is fit around a center cone on the disc-clamping side, thereby centering the disc. For the inner diameter/outer diameter machining of a disc, the Thomson method is often used, in which the disc is cut with a thin blade such as a razor.
Patent Document No. 1 describes a method in which the inner diameter/outer diameter machining of a disc is done with laser. In the cut-out process using laser, an array of pits provided on the substrate are detected to determine the center point in the array of pits, thereby positioning the inner diameter and the outer diameter at predetermined distances from the center point.
While positioning is done in Patent Document No. 1 based on an array of pits on the substrate, Patent Document No. 2 discloses providing a mark for positioning.
Patent Document No. 3 describes a method in which in order to eliminate the play with a disc supporting portion of a recording/reproducing apparatus which occurs because the thickness of the disc is small, a slope is provided toward an end portion of the inner hole of the disc to match the slope of the disc supporting portion.